A key requirement on Long Term Evolution (LTE) for radio access as defined in 3GPP is frequency flexibility for transmissions between a radio base station and a mobile terminal over a radio link. For this purpose, carrier bandwidths between 1.4 MHz and 20 MHz are supported, as is both Frequency Division Duplex (FDD) and Time Division Duplex (TDD), so that both paired and unpaired frequency spectrum can be used. For FDD, the downlink (DL), i.e. the link from a base station to a mobile terminal, and uplink (UL), i.e. the link from a mobile terminal to a base station, use different frequencies so called “paired frequency spectrum” and can hence transmit simultaneously. For TDD, uplink and downlink use the same frequency “unpaired” frequency spectrum” and can not transmit simultaneously. Uplink and downlink can however share the time in a flexible way, and by allocating different amounts of time, such as the number of subframes of a radio frame, to uplink and downlink, it is possible to adapt to asymmetric traffic and resource needs in uplink and downlink.
The above asymmetry also leads to a significant difference between FDD and TDD. In LTE time is structured into radio frames of 10 ms duration, and each radio frame is further divided into 10 subframes of 1 ms each. Whereas for FDD, the same number of uplink and downlink subframes is available during a radio frame, for TDD the number of uplink and downlink subframes may be different. One of many consequences of this is that in FDD, a mobile terminal can always send feedback in response to a data packet in an uplink subframe subject to a certain fixed processing delay. In other words, every downlink subframe can be associated with a specific later uplink subframe for feedback generation in way that this association is one-to-one, i.e. to each uplink subframe is associated with exactly one downlink subframe. For TDD however, since the number of uplink and downlink subframes during a radio frame may be different, it is in general not possible to construct such a one-to-one association. For the typical case with more downlink subframes than uplink sub-frames, it is rather so that feedback from several downlink subframes requires to be transmitted in at least one of the uplink subframes.
In Evolved Universal Terrestrial Radio Access (E-UTRA), a radio frame of 10 ms duration is divided into ten subframes, wherein each subframe is 1 ms long. In case of TDD, a subframe is either a special subframe as described below or assigned to uplink or downlink, i.e., uplink and downlink transmission cannot occur at the same time. Furthermore, each 10 ms radio frame is divided into two half-frames of 5 ms duration where each half-frame consists of five subframes.
The first subframe of a radio frame is always allocated to downlink transmission. The second subframe is a special subframes and it is split into three special fields, a downlink part DwPTS, a Guard Period (GP) and an uplink part UpPTS, with a total duration of 1 ms.                UpPTS is, if so configured, used for transmissions of sounding reference signals in the uplink and, if so configured, used for reception of a shorter random access preamble. No data or control signalling can be transmitted in UpPTS.        GP is used to create a guard period between periods of downlink and uplink subframes and may be configured to have different lengths in order to avoid interference between uplink and downlink transmissions. The length is typically chosen based on the supported cell radius.        DwPTS is used for downlink transmission much like any other downlink subframe with the main difference that it has shorter duration.        
Different allocations of the remaining subframes to uplink and downlink transmission are supported, both allocations with 5 ms periodicity in which the first and second half-frame have identical structure, and allocations with 10 ms periodicity for which the half-frames are organized differently. For certain configurations the entire second half-frame is assigned to downlink transmission. In case of 5 ms periodicity, the ratio between downlink and uplink may e.g. be 2/3, 3/2, 4/1 (regarding DwPTS as a full normal downlink subframes), etc. In case of 10 ms periodicity, the ratio between downlink and uplink may e.g. be 5/5, 7/3, 8/2, 9/1 etc.
In the downlink of E-UTRA, OFDM, Orthogonal frequency-division multiplexing, with a subcarrier spacing of 15 kHz is used. Depending on the configured cyclic prefix length, a 1 ms subframe contains either 12 or 14 OFDM symbols in time. The term resource block is also used to refer to the two-dimensional structure of all OFDM symbols within a half subframe, a slot, times 12 consecutive subcarriers in the frequency domain. The downlink part of the special subframe, DwPTS, has a variable duration, and can assume lengths of 3, 9, 10, 11 or 12 OFDM symbols for the case with normal cyclic prefix, and 3, 8, 9 or 10 symbols for the case with extended cyclic prefix.
In the uplink of E-UTRA, SC-FDMA, Single Carrier Frequency Division Multiple Access, also referred to as Discrete-Fourier-Transform (DFT)-pre-coded OFDM, is used. The underlying two-dimensional (time and frequency) numerology is the same in terms of subcarrier spacing, cyclic prefix lengths and number of OFDM symbols. The major difference is that modulated data symbols to be transmitted in certain OFDM symbols are subject to a DFT and the outputs of the DFT are mapped to the subcarriers.
In order to improve performance of transmission in both the downlink and uplink direction, LTE uses Hybrid Automatic Repeat Request (HARQ). The function of this mechanism for downlink transmission is discussed below.
The basic idea of HARQ is that after receiving data in a (part of a) downlink subframe the terminal attempts to decode it and then reports to the base station whether the decoding was successful (ACK, acknowledgement) or not (NAK, negative acknowledgement). In case of an unsuccessful decoding attempt the base station thus receives a NAK in a later uplink subframe, and can retransmit the erroneously received data.
Downlink transmissions can be dynamically scheduled, i.e. in each downlink subframe the base station transmits control information on which terminals are to receive data and upon which resources in the current downlink subframe. Such a control information message to a terminal is referred to as a downlink assignment. A downlink assignment thus contains information to which terminal the assignment is intended to and also information to the intended terminal about in which resources, for example how many and which resource blocks, data will be sent, and also information necessary for the terminal to decode the subsequent data, such as modulation and coding scheme. Resources here comprise some set of resource blocks. This control signalling is transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and data is sent in the remaining part of the subframe. The data sent to a terminal in a single downlink subframe is referred to a transport block and an ACK/NAK is sent in response to the transmission.
A terminal will thus listen to the control channels in the downlink subframes, and if it detects a downlink assignment addressed to itself, it will try to decode the subsequent data. It will also generate feedback in response to the transmission, in the form of an ACK or a NAK depending on whether the data transport block was decoded correctly or not. Furthermore, from the control channel resources on which the assignment was transmitted by the base station, the terminal can determine the corresponding uplink control channel resource. Hence, a downlink control channel is associated with an uplink control channel resource, and on a downlink control channel, a downlink assignment can be transmitted. In each DL subframe, several control channels may be transmitted and hence several users may get assigned data in uplink and downlink. Additionally, a UE may listen to several control channels.
For E-UTRAN FDD the terminal will in response to a detected downlink assignment in subframe n attempt to decode the transport block(s) sent to the terminal in subframe n and send an ACK/NAK report in uplink subframe n+4. For the case with so-called Multiple Input Multiple Outout (MIMO) multi-layer transmission, two transport blocks are transmitted in a single downlink subframe, and the terminal will respond with two ACK/NAK reports in the corresponding uplink subframe.
The assignment of resources to the terminals is handled by the scheduler, which takes into account traffic and radio conditions so as to use the resources efficiently while also meeting delay and rate requirements. Scheduling and control signaling may be done on a subframe to subframe basis. Typically, each downlink subframe is scheduled independently of others.
As described above, the first step for a terminal to receive data from the base station in a downlink subframe is to detect a downlink assignment in the control field of a downlink subframe. In the case that the base station sends such an assignment but the terminal fails to decode it, the terminal obviously cannot know that is was scheduled and will hence not respond with an ACK/NAK in the uplink. This situation is referred to as a missed downlink assignment. If the absence of an ACK/NAK can be detected by the base station, it can take this into account for subsequent retransmissions. Typically the base station should at least retransmit the missing packet, but it may also adjust some other transmission parameters.
Since downlink assignments can be given independently across downlink subframes, a terminal may be assigned downlink transmissions in multiple downlink subframes that are all to be acknowledged in a single uplink subframe. Hence, the uplink control signalling needs to support, in some way, feedback of ACK/NAKs for downlink transmissions in multiple downlink subframes from a terminal in a given uplink subframe.
One way is to allow the terminal to transmit multiple individual (for each downlink transmission in each downlink subframe) ACK/NAK bits in a single uplink subframe. Such protocols have however worse coverage than transmission of a one or two ACK/NAK reports. To improve control signaling coverage and capacity, it is possible to perform some form of compression, or bundling, of ACK/NAKs, referred to as ACK/NAK bundling. This means that all ACK/NAKs that are to be sent in a given uplink subframe are combined into a smaller number of bits, such as a single ACK/NAK report. As an example, the terminal can transmit an ACK only if the transport blocks of all the downlink subframes were received correctly and hence to be acknowledged. In any other case, meaning that at a NAK for at least one downlink subframe is to be transmitted, a combined NAK is sent for all downlink subframes. As described above, to each uplink subframe in TDD a set of downlink subframes can be associated rather than a single subframe as in FDD, for which downlink transmissions are to be given ACK/NAK response in the given uplink subframe. In the context of bundling this set is often referred to as the bundling window.
Another advantage of bundling is that it allows reusing the same control channel signaling formats as for FDD, independently of the TDD uplink/downlink asymmetry. The disadvantage is a possibly small loss in downlink efficiency. If the base station receives a NAK it cannot know how many and which downlink subframes were received erroneously and which were received correctly. Hence it may need to retransmit all of them.
A problem with ACK/NAK bundling is that a terminal may miss a downlink assignment, which may not be indicated in the bundled response. For instance, assume that the terminal was scheduled in two consecutive downlink subframes. In the first subframe the terminal misses the scheduling downlink assignment and will not be aware that it was scheduled, while in the second subframe it did successfully receive the data. The terminal will, as a result, transmit an ACK, which the base station will assume holds for both subframes, including data in subframe the terminal was not aware of. As a result, data will be lost. The lost data needs to be handled by higher-layer protocols, which typically takes a longer time than HARQ retransmissions and is less efficient. In fact, a terminal will not transmit any ACK/NAK in a given uplink subframe only if it missed every downlink assignment that was sent during the bundling window associated with the uplink subframe.
Thus, a missed downlink assignment will in general result in block errors that need to be corrected by higher-layer protocols, which in turn has a negative impact on performance in terms of throughput and latency. Also, increasing the delay may cause undesirable interactions with TCP based applications.